The present invention relates to consolidation and densification of metal alloy powders.
The field of the invention comprises production of metal parts from powders to achieve essentially full density, i.e., 98-100% of theoretical density. The prior art approaches to such production are troubled by high capital and operating costs of high temperature and/or high pressure production equipment, and problems of grain growth during production.
In many applications of sintered powder metallurgy (PM) parts, essentially full density is required to meet requirements for fatigue strength, ductility, ultimate tensile strength, impact strength, corrosion resistance and hardness which are typically exhibited by cast, forged or wrought parts. Such fully dense PM parts heretofore have been made using powder metallurgy methods involving high temperature vacuum sintering which is considered expensive and difficult to control. Generally, vacuum sintering yields a wide uncontrolled distribution of sintered densities between 94 and 98.5 percent of theoretical. Another approach which has been used with the alloys Ti-6Al-.sub.4 V, Monel, M-2, 4600, 4650, 316L, Stellite 21 and Stellite 6 comprises processing to full density by first pressing suitably prepared alloy metal powders and sintering them to a condition of closed porosity, usually about 92 percent of theoretical density, followed by hot isostatic pressing ("HIP") at 15,000 psi and temperatures 100.degree. to 300.degree. C. below the sintering temperatures for closed porosity.
With this method, the problems of achieving full density were essentially solved. However, the cost of high pressure HIP equipment was a major deterrent to using this method in production.
Processes, and equipment, per se, for sintering, die pressing and/or isostatic pressing, per se and in hybrid combinations are well known for consolidation and densification of powder compacts to over 95% of theoretical density. The compact so treated may be a simple geometric form, such as a cube, or a complex shaped part.
It is the object of the invention to provide technical and economical feasibility for metal alloy parts PM production through a modified form of sintering described herein overcoming the above problems.